Automatic reverberation control



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United States Patent O 3,221,092 AUTUMATIC REVERBERATION CONTROL Max W. Rogers, Melrose Park, Ill., assigner to Admiral Corporation, Chicago, Ill., a corporation of Delaware Filed Apr. 3, 1961, Ser. No. 100,036 9 Claims. (Cl. 84-1.24)

This invention relates generally to means for artificially augmenting the reverberative content of musical signals to create an illusion of presence for the listener. In particular the invention concerns means for automatically controlling the artificially augmented reverberative content of such musical signals responsive to changes in the setting of the level or loudness control of the reproducing system.

It does not require a musically trained ear to detect the vast difference between listening to a live concert hall performance of a musical selection and listening to the same musical selection in ones home, via recordings or rceived radio transmission. Many factors must be considered in accounting for this difference. Assuming good recording (or transmission) and reproducing equipment, chief among these factors are the level at which the reproduced music is played, the spaciousness of the room, and the acoustic properties of the room, i.e., whether the room is acoustically live or acoustically dead Since all of these factors vary with respect to each individual installation, it has been desirable to develop means for artificially augmenting the reproduced music to create for the listener the illusion that the music is being played in a concert hall. -To this end numerous reverberation units have been developed. These units in general comprise means for selecting a portion of a normal signal, means for delaying the selected portion and means for mixing the delayed portion with the normal signal. Thus a sort of echo effect is achieved which markedly adds to listening pleasure, especially at low listening levels.

As the degree of reverberation desired also varies with individual tastes and the type of program music employed, most reverberation systems in use today include some type of control for setting the desired reverberation level. These controls generally require readjustment whenever the main level or loudness control of the reproducing system is varied. This follows since the reverberation effect is most noticeable and desirable at low to medium listening levels. If the amount of reverberation is not decreased at loud listening levels, great distortion, and consequent lack of clarity in the reproduced music, results.

One such system employs means whereby, as the normal acoustic output level of the reproducing apparatus is increased, the amount of reverberation is also increased, but at a lesser rate. The system of the invention is based upon the premise that for maximum realism and clarity of reproduced sound, substantially zero artificial reverberation should be present when the acoustic level of the reproduced music is near maximum. It has been noted that in all but acoustically dead rooms, the illusion of presence is readily obtainable by playing the musical selection at a fairly loud level without artificial reverberation,

A system fulfilling the above may be achieved by providing another control for the reverberative portion of the system which limits either the signal input to the reverberation unit, or the signal output therefrom, as the level control of the reproducing system is turned up. In such a system the additional control would be ganged to the volume control of the main reproducing unit to provide for simultaneous operation.

The system of the invention accomplishes this result without requiring additional controls or added circuitry. In accordance with the preferred embodiment of the in- 3,221,092 Patented Nov. 30, 1965 ICC vention the signal input to the reverberation unit is connected across a fixed portion of the main level control and the signal output from the reverberation unit is connected to the movable tap on this level control. As will be described more fully hereinafter, as the movable tap on the level control is raised to increase the acoustic output of the system, the reverberative component increases to a maximum for a first range of level control settings and then decreases over a subsequent range of level control settings. By proper selection of circuit parameters, maximum reverberative content is obtained at approximately midrotation of the level control and substantially zero reverberative content is obtained at the lmaximum setting of the level control.

Accordingly it is an object of this invention to provide a sound reproducing system employing artificial reverberation, in which the reverberative content in the acoustic output is automatically varied as the acoustic output level of the system is varied.

Another object of this invention is to provide means for automatically controlling the reverberative content of a musical signal as a function of the main amplifier level control setting.

A further object of this invention is to provide an automatic reverberation control for a sound reproducing system with artificial reverberation, which control functions to substantially eliminate the reverberative component from the acoustic output of the system level control.

Still another object of this invention is to provide an automatic reverberation control system in which the input to the reverberation unit is connected across a fixed portion of the main amplifier acoustic output level control and the output of the reverberation unit is connected across a variable portion of the main amplifier acoustic output levelcontrol.

A st-ill further object of this invention is to provide a musical augmentation system employing reverberation in which the reverberative component in the system acoustic output is increased over a first range of increasing system output level and is decreased over a subsequent range of increasing system output level.

Further objects of this invention will become apparent upon a reading of the following specification in conjunction with the drawings in which;

FIG. 1 is a schematic diagram of a sound system employing the invention;

FIG. 2 is an idealized curve depicting the operation of the circuit of FIG. 1;

FIG. 2a is a simplified schematic diagram of a portion of FIG. l; and

FIG. 3 is a modification of the circuit of FIG. 1 incorporating the invention in a stereophonic environment and utilizing a loudness control in place of the level control of FIG. l.

Referring now to FIG. l, a source of electric musical signals 5 may comprise a source of recorded material or means for receiving and demodulating a transmitted signal. Amplifying stage 10 is conventional and contains a vacuum tube 11 having an anode 12, a cathode 13 and a control grid 14. Anode 12 is connected through a load resistor 15 to a source of positive potential B-|. Responsive to a signal appearing at control grid 14, a corresponding, though larger, signal is developed across load resistor 15. This larger signal is coupled by a coupling capacitor 16 to a level or volume control, comprising a resistance element 17 and a movable tap 18. Movable tap 18 is coupled through a coupling capacitor 22 to amplifier 23 which contains additional conventional amplification stages. The output of amplifier 23 drives speaker 24 which converts the electric signals into corresponding acoustic signals. Amplifier 23 may also include listener preference controls, such as treble and bass controls.

The signal appearing across resistance element 17 is also connected to a series network comprising resistor 19, capacitor 20 and resistor 2l. While the series network shown is connected to the top of resistance element 17, it will be appreciated by those skilled in the art that the network may be connected to a point physically on resistance element 17 or to any point in the system where the signal level is determined only by the program source. The signal developed across resistor 21 is amplified in amplifying stage 25 which includes a vacuum tube 26 having an anode 27, a cathode 28 and a control grid 29. An amplified signal is developed across load resistor 30, connected between B-land the anode of tube 26. Cathode resistor 31 provides proper bias for tube 26.

Depending upon the particular type of reverberation unit employed, amplifying stage 25 may or may not be required. In the type of reverberation unit utilized in the embodiment chosen to illustrate the invention, electrical signals are fed to a transducer which converts these signals into mechanical signals. The resulting mechanical signals are delayed through delay line means and are subsequently picked up by another transducer which converts the mechanical signals back into corresponding electrical signals. These delay line means introduce a time delay, between input and output, to signals appearing at the input. These units, such as reverberation unit 33, are well known in the art and need no detailed explanation here. Most of these units have a substantial power loss and hence require amplification of the signal before entering the unit and amplification of the signal after leaving the unit.

Capacitor 32. couples the signal to the input of the reverberation unit. The output of the reverberation unit is coupled through a capacitor 34 to an amplifying stage 36. This stage comprises a vacuum tube 37 having an anode 38, a cathode 39 and a control grid 4l). A load resistor 41 is connected between B+ and anode 38. Grid leak resistor 35 develops operating bias for tube 37. The output of tube 37 is coupled by a coupling capacitor 42 to a potentiometer comprising a fixed resistance 43 and a movable tap 44. This potentiometer is incorporated to allow the listener to determine how much reverberative component he wishes in the system output. Tap 44 is coupled through a capacitor 45 and a grid leak resistor 46 to grid 54 of vacuum tube 51 in amplifying stage 50. Tube 50 also has an anode 52, a cathode 53 and a cathode bias resistor 4S. Responsive to a signal on grid 54, a correspondingly larger signal is developed across load resistor 47, which larger signal is coupled, through coupling capacitor 49 and a large resistor 55, to tap 18 on the main level control in the system. Reverberation unit 33, and amplifying stages 36 and 50, respectively are surrounded by a dashed line rectangle 60. This is for convenience of description in a later embodiment of the invention.

To recapitulate, signals from signal source are amplified in amplifying stage 1t) and coupled to the main level control of the sound system. The position of the movable tap on the level control determines how much of the signal will appear at the input of amplifier 23 and consequently determines the acoustic output level of the system. A portion of the signal across the level control is coupled to amplifying stage 30 which feeds block 60, containing the reverberation unit and additional amplifying stages. The net effect of the components in block 60 is to introduce a time delay between the direct signal and the output of block 60. The delayed signal is coupled to the movable tap 18 on the main level control.

The effect of this connection is quite striking. With the movable tap 18 set at minimum, that is at the lower end of resistor 17, no signal, either direct or delayed, is fed to amplifier 23 and consequently no system acoustic output is obtained. As the movable tap 1S on the level control is raised, both the direct and the reverberative signal outputs increase. At approximately mid-rotation of the level control the reverberative signal in the system output reaches a maximum. Further movement of the tap toward maximum volume, results in corresponding increases in the direct signal output but corresponding decreases in the reverberative signal output.

FIG. 2 shows a series of curves illustrating this relationship. The ordinate represents the signal level to amplifier 23 and the abscissa represents the main level control setting. The level control is assumed for illustrative purposes to be linear. At the loud setting curves B and C indicate substantially zero signal level, whereas curve A indicates a high signal level. At a medium setting curves B and C have reached maximum signal level points. Curve A represents the direct signal level input to amplifier 23, and as expected, continues ascending as the setting of the level control is raised. Curve B represents the reverberative component signal level input to amplifier 23 (assuming that tap 44 on the reverberation potentiometer is at its maximum setting). Curve C illustrates the reverberative component signal level to amplifier 23 when tap 44 on the reverberation potentiometer is at a medium setting. From this curve (curve C) it is seen that the reverberation potentiometer determines the maximum amount of reverberative component present in the total signal fed to amplifier 23.

The curves graphically illustrate the operation of the invention. Over a portion of the range where the direct signal level to amplifier 23 is increasing (and consequently the acoustic output of the system is increasing) the reverberative component begins to decrease. In these curves the reverberative component is seen to be zero at the maximum setting of the level control. By appropriate selection of resistors 17, 55, and 15, and tube 11 it can be shown that the occurrence of the maximum point on the reverberative component curve may be shifted with respect to the position of movable tap 18 on resistor 17. The system may be designed such that the maximum reverberative component occurs at seventy-five percent rotation of the main level control arm, in which case the reverberative component would not quite fall to zero at the maximum level control setting. This and other combinations will be recognized as variations of the idealized arrangement depicted by the curves of FIG. 2, in which at maximum loudness setting the reverberative component is substantially zero.

Reference to FIG. 21a may help to clarify the operation of this circuit although it should be understood that the theory of operation herein presented is only for explanatory purposes and no averment of accuracy of the theory is made. Resistance 17 is connected to ground at one terminal. Its other terminal is returned to ground through a parallel combination of resistors. This representation is for signal purposes only. Capacitor 16 has therefore been omitted since, being a signal coupling capacitor, it has negligible impedance at signal frequencies. Likewise the components (not shown) for developing the B-lpotential have substantially zero impedance for signal frequencies. Thus the parallel combination of resistors comprises load resistor 15 and a resistance, labelled tube resistance, which is dependent upon the characteristics of tube 1l. This parallel combination of resistors has a value low in comparison with resistance 17 and resistor 55, and consequently, when the movable tap 18 is moved to either end of resistor 17, substantially the entire output voltage from block 6l) appears across resistor 55. For other positions of the movable tap, some of the output voltage from block 60 is developed across resistance 17. Thus, by this voltage divider action, the maximum reverberative voltage for amplifier 23 is obtained with tap 18 approximately midway between the ends of resistance 17. With the tap at either end of resistance 17, substantially zero reverberative voltage is available for amplifier 23.

FIG. 3 is a schematic diagram of the invention adapted for use in a stereophonic sound environment. A Source of stereo signals having two separate output terminals coupled to two signal translation channels is shown. For descriptive purposes, it will be assumed that the upper translation channel is the left stereophonic channel and the lower translation channel the right stereophonic channel. The reverberation unit and its associated amplifying stages is common to both channels. Only the lower or right channel operation will be described, it being remembered that the upper or left channel operates in a similar manner.

The output from source 6 is amplified in amplifying stage 110 and develops a potential across loudness control 170. Loudness control 170 comprises a resistance 171 and a movable tap 172 thereon. The upper poltion of the :ontrol is bridged by a capacitor 173 and the lower portion is bridged by the series combination of resistor 174 1nd capacitor 175. The loudness control adjusts the frequency characteristic of the amplifier to compensate for the frequency response characteristics of the human ear at low listening levels. Thus, when the control is set for low loudness levels, the bass and treble content of the signal is augmented.

Movable tap 172 on loudness control 170 is coupled through capacitor 122 to the main right channel amplifier 123 where the signal is further amplified and used to feed a loud speaker 124. The signal developed across resistance 171 of loudness control 170 is also fed to the series combination of resistor 119, capacitor 120 and resistor 121. The junction of resistor 121 and capacitor 120 is connected to the control grid of vacuum tube 126 in amplifying stage 125. This amplifying stage comprises tube 126 having an anode 127, a cathode 128 and control grid 129. The right channel signal is thus amplified in amplifying stage 125 and a corresponding signal is developed across load resistor 30.

Load resistor 3i) is common to both amplifying stages 125 and 25. It will be noted that amplifying stage 25 develops the amplified left channel voltage and consequently, as will be readily appreciated by those skilled in the art, a mixing of the two individual channel signals occurs across load resistor 30. In other words, resistor 30 sees lan L-i-R signal, where L represents the left channel signal and R represents the right channel signal. This combined signal is coupled via capacitor 32 to the reverberation system 6d, which functions in the manner previously described.

The output of reverberation system 60 is coupled via capacitor 49 to both the left and right channel loudness controls. It is coupled to movable tap 72 through resistor S5, and to movable tap 172 through resistor 155. Thus the reverberative signal, which is a combination of the left and right channel signals, and which has been delayed relative to the main signals, is now injected equally into both channels.

The individual loudness controls, which have their respective movable taps ganged together for simultaneous operation, operate in substantially the manner described for the level control of FIG. l. As the individual movable taps are raised toward maximum settings, the output of reverberation system 60 is substantially shorted through the parallel combinations of the internal resistance of tube 11 and its load resistor 12, and the internal resistance of tube 111 and its load resistor 115, respectively. There is a slight effect introduced by the frequency compensating networks connected to the loudness controls which results in the curves of reverberative signal versus loudness control setting to be displaced somewhat from the idealized ones shown in FIG. 2a. However the effect is small and not noticeable to the ear.

In its preferred form the invention envisions a loudness or level control which is fed from a source having a low internal-impedance, such as a vacuum tube. However, the system will operate satisfactorily if a high impedance source is used in conjunction with a tone compensated 6 level control (i.e., a loudness control), such as those illustrated in FIG. 3.

Such a system may be envisioned by eliminating amplifying stages 10 and 110, from FIG. 3 .and having the loudness controls 70 and 170 coupled directly to the left and right channel outputs, respectively, of a high impedance stereo source, such as a ceramic cartridge. tem it has been found desirable to increase the values of resistors 55 and 155, respectively, and to make appropriate changes in the respective t-one compensation networks. With the exercise of appropriate skill in the selection of component values of the loudness control, a reverberation signal versus loudness control setting reaching a maximum at about one-half to three quarters rotation of the loudness control, may be achieved. In such a system however, zero reverberation signal is not realized at maximum loudness since the compensating networks cannot have zero impedance. This results from the fact that the compensating network is passive and presents the same impedance to the main signal as -it does to the reverberative signal, whereas the vacuum tube is an active element with respect lto the main signal and a passive element with respect to the reverberative signal. However a system employing a high impedance source has operated satisfactorily with `a noticeable decrease in reverberation at near maximum loudness control settings.

Once again it is pointed out that the curves of FIG. 2a are `for a linear level control, i.e., one which has a relatively fixed ratio of resistance to .angle of rotation. If tapered controls are used, that is the types which do not change resistance equally for equal changes in angles of rotation, then the abscissa of the FIG. 2a curves would be in terms of resistance and not rotation. Those skilled in the art are familiar with these non linear controls.

What has been described is a novel circuit arrangement for automatically adjusting the level of the artificially produced reverberative component of a musical signal in accordance with the setting of the main level control of the sound system. While specific components have, of necessity, been illustrate-d in the various embodiments, it will be appreciated that those skilled in the lart may readily substitute other components of like function, such as transistors for tubes. These features are not considered of the essence of the invention which centers about the connection of the reverberation system input to `a fixed signal point in the amplifiying system and the connection of the reverberation system output t-o the movable 4arm of the system level or loudness control. The scope of this invention is limited only as defined in the appended claims.

What is claimed is:

1. In combination; an amplifier having a first amplifying section and a second amplifying section; a signal source coupled to said first amplifying section; a level control connected between said first and said second amplifying sections, said level control including a resistance element and a movable tap thereon for varying the signal level input to said second amplifying section; a transducer coupled to said second amplifying section for converting electrical `signals into corresponding acoustical signals; a reverberation unit having .an input circuit coupled across said resistance element and an output circuit coupled to said movable tap on said resistance element; said reverberation unit including means for accepting at least a portion of the signal in said first amplifying section and introducing a delay in the `accepted signal between said input circuit and said output circuit, whereby variations in the level of the reverberated signal fed to said second amplifying section occur in accordance with a nonlinear function of the setting of said movable tap.

2. In combination in a signal translation channel; a direct signal source; a load; an amplifier coupled between said source and said load for amplifying signals emanating from said source; a level control in said amplifier for varying the direct signal-level therein, said level control In such a sys.

including a resistance eiernent with a movable tap thereon; a delay circuit coupled across said resistance elenient, said delay circuit being effective to introduce a delay to signals impressed therein and thereby producing a reverberative component; means in said de-lay circuit for setting the reverberative component level thereof; means coupling the output of `said delay circuit to said movable tap on said level control; said reverberative component being combined with the direct signal in sa-id amplifier to provide a reverberated signal, the maximum amount of reverberative component present in said amplifier being dependent upon the setting of said means in said delay circuit; the ratio between said reverberative component and direct signal present in said amplifier vary-ing in .accordance with the setting of said level control and decreasing over at Ileast a portion of said level control settings where the direct signal level in said amplifier is increasing.

3. A sound reproducing system comprising; a signal source; an amplifier coupled to said source amplifying signals received therefrom; a transducer coupled to said amplifier converting output signals thereof into corresponding acoustic signals; a reverberation unit having an input and an output, said unit introducing a time delay between said input and said output to signals passing therethrough; a tone compensated level control connected between said signal source and said amplifier, said tone compensated level control having a fixed point and a movable element for varying the signal level to the amplifier; means coupling said fixed point on said tone compensated control to the input of said reverberation unit; means coupling the output of said reverberation unit to said movable element of said control whereby the magnitude of the reverberated signal fed to said amplifier varies with the setting of said level control.

4. In combination; a signal source; an amplifier and transducer amplifying signals from said source and converting the .amplified signals into corresponding acoustic signals; a level control coupled between said signal source and said amplifier for controlling the level of signals fed to said amplifier, said level control including a resistance element having a mov-able tap thereon; a delay circuit connected across a fixed portion of said resistance element; said delay circuit including means delaying signals passing therethrough with respect to signals emanating from said source, and reintroducing said delayed signals to the movable tap on said level control, said level control and the effective impedance of said signal source forming a load for said delayed signal whose magnitude is dependent upon the setting of said tap; said amplifier being fed signals from said source and delayed signals from said delay circuit, said movable tap increasing the level of said delayed signals fed to said amplifier over a first range of tap movement and decreasing the level of said delayed signals fed to said amplifier over a subsequent second range of tap movement.

5. In combination in a sound system for artifically adding a reverberative component to signals therein; a source of normal signals; a source of reverberative signals; a load; an amplifier coupled between said sources and said load amplifying both said normal and said reverberative signals; a resistance coupled across said source `of normal signals; a movable tap on said resistance, .said movable tap being coupled to said amplifier and varying the normal signal level to said amplifier as a direct function of the setting thereof; said source of reverberative signals including means accepting at least a portion of the output `of said normal signal source, developing said reverberative component therefrom, and introducing said reverberative component to said movable tap; said normal signal level to said amplifier increasing as said movable tap is moved across said resistance in one direction; means, including said source of signals, said resistance and said tap, providing a load for said reverberative signals whose magnitude is dependent upon the position of said movable tap; whereby said reverberative component level to said arnplifier increases as said movable tap is moved in said one direction over a first portion of said movement and decreases as said movable tap is moved in said one direction over a second portion of said movement.

6. In combination; a signal source generating two separate though related musical signals; a pair of signal translation channels each including an amplifying stage amplifying respective ones of said two signals; a level control in each said translation channel varying the signal amplitude in its respective channel, said level controls including respective variable impedance elements arranged to be operated simultaneously; a common reverberation channel coupled to both said translation channels; said reverberation channel including means producing a third signal by mixing said two signals and introducing a time del-ay thereto; means coupling said third signal across said variable impedance elements 0f each of said level controls, whereby the amplitude of said third signal in said translation channels varies with variations in the settings of said level controls and load circuit means coupled to said translation channels.

'7. In a musical reproducing system of the type having a source of electrical musical signals, an amplifier amplifying the electrical signals, a speaker coupled to said aniplifier converting the amplified electrical signals into corresponding acoustical signal, said amplifier including a level control having a fixed .portion and `a variable tap thereon for varying the amplitude of the electrical signals in said amplifier, and a reverberation system artificially augmenting the `acoustical signals by selecting a portion of said electrical sign-als and delaying them with respect to the unselected signals, the improvement comp-rising; means coupling said selected electrical signals to said reverberation system from across the fixed portion of said level control such that said amplifier signal amplitude is unaffected by setting of said level control; and means coupling the delayed signals to said variable tap on said level control, the effective impedance of said source of musical signals, said level control and said variable tap cooperating to provide a variable load for said delayed signals.

8. In combination; a high impedance source of electrical musical signals; an amplifier amplifying said electrical signals, a transducer coupled to said amplifier converting said electrical musical sign-als to acoustical musical signals; a variable impedance coupled between said source and said amplifier for varying the level of said electrical signals in said amplifier, said variable impedance means comprising a loudness control includir,7 a resistance element with a movable tap thereon, and a resistance-capacitance tone compensation network coupled across a portion of said resistance element; means artificiai augmenting said acoustical musical signals comprising delay means having an input connected across said resistance element and an output coupled to said movable tap on said resistance element, said delay means accepting at least a portion of said electrical signals from said source and introducing a time delay between said accepted portion and the unaccepted portion of said signals.

9. In combination; a source of electrical musical signals comprising a first amplifier including an electron discharge device having an anode, `a cathode, and a control grid; a second amplifier amplifying said electrical signals, a transducer coupled to said second amplifier converting said electrical musical signals into corresponding acoustical musical signals; variable impedance means, comprising a fixed resistance element coupled between said anode and said cathode and a movable tap on said resistance element, for varying the level of said electrical signals in said second amplifier; means artificially augmenting said acoustical musical signals comprising delay means having an input connected across at least a portion of said fixed resistance element and an output coupled to said movable tap, said delay means accepting at least a portion of said electrical signals from said source and introducing a time 9 10 delay between the accepted portion and the unaccepted OTHER REFERENCES @0mm of said Signals' Radio-Electronics, v01. XXXI, No. 8, August 1960,

References Cited by the Examiner page 43' UNITED STATES PATENTS 5 DAVID I. GALVIN, Primary Examiner. 1,853,286 4/1932 Round 1791.6 CARL W ROBINSON, Examiner, 2,557,318 6/1951 Shirk 333-28 2,872,515 3/1959 Goldmark 84 1.24 K- B KELLER, ASSI-Smm Examiner- 

1. IN COMBINATION; AN AMPLIFIER HAVING A FIRST AMPLIFYING SECTION AND A SECOND AMPLIFYING SECTION; A SIGNAL SOURCE COUPLED TO SAID FIRST AMPLIFYING SECTION; A LEVEL CONTROL CONNECTED BETWEEN SAID FIRST AND SAID SECOND AMPLIFYING SECTIONS, SAID LEVEL CONTROL INCLUDING A RESISTANCE ELEMENT AND A MOVABLE TAP THEREON FOR VARYING THE SIGNAL LEVEL INPUT TO SAID SECOND AMPLIFYING SECTION; A TRANSDUCER COUPLED TO SAID SECOND AMPLIFYING SECTION FOR CONVERTING ELECTRICAL SIGNALS INTO CORRESPONDING ACOUSTICAL SIGNALS; A REVERBERATION UNIT HAVING AN INPUT CIRCUIT COUPLED ACROSS SAID RESISTANCE ELEMENT AND AN OUTPUT CIRCUIT COUPLED TO SAID MOVABLE TAP ON SAID RESISTANCE ELEMENT; SAID REVERBERATION UNIT INCLUDING MEANS FOR ACCEPTING AT LEAST A PORTION OF THE SIGNAL IN SAID FIRST AMPLIFYING SECTION AND INTRODUCING A DELAY IN THE ACCEPTED SIGNAL BETWEEN SAID INPUT CIRCUIT AND SAID OUTPUT CIRCUIT, WHEREBY VARIATIONS IN THE LEVEL OF THE REVERBERATED SIGNAL FED TO SAID SECOND AMPLIFYING SECTION OCCUR IN ACCORDANCE WITH A NONLINEAR FUNCTION OF THE SETTING OF SAID MOVABLE TAP. 